Pretty much any electrical device requires a hot and a neutral, the technical explanation gets a little in depth but in simple terms you you a hot(+) and a neutral(-) to power a device. Do not confuse a home ground as a neutral(-) because it’s not.
When power comes from your power company it comes on 3 lines, generally. 2 hots (+'s) and a neutral(-). Devices that require 110v use only one of the hots and devices that use 220v use both hots. All devices use the neutral. The ground doesn’t come from your power company, it normally comes from a ground rod driven into the earth outside your home.
Also when I say neutral(-) the minus isn’t really a negative, AC Power isn’t really a negative. I was just using it in the example because most people understand DC Power as + and -.
Many people will tell you that you can fake the neutral with you home ground, this is a very bad idea and even when it does work can be dangerous.
So to answer your question a Wemo,Z-wave,Zigbee switch requires constant power hot and neutral to power the radio and then uses a relay to turn the load on and off, this is why a normal switch doesn’t need a neutral and the smart switch does. The device your powering has them both but the switch location usually only has the hot wire.
This has been a real pain for me in home automation because I do not have neutrals in some locations and in most locations I have very old and stiff knob and tube wiring.
Something I’ve learned since writing this post is there are some z-wave dimmers that do not require a neutral. Most will not work with CFL bulbs, or some LED bulbs. This is because the dimmer allows a small amount of power to flow through at all times and most LEDs and CFL’s see that power and will not fully shut off. The other issue is that using the dimmer with CFL’s and LED’s will be erratic at best. There are some LED’s that work, best way to know if see if they work with Dimmers.